This study analyzed the associations between physical activity (PA) and motor proficiency (MP) to determine what level of moderate PA might help avoid low MP in 8-10-year-old Brazilian children. We assessed MP of 98 children using the Bruininks-Oseretsky Motor Proficiency, Second Edition Short Form (BOT-2 SF), and we assessed PA using the GT3X + accelerometer. We analyzed data using means, standard deviations, frequency distribution, spearman correlation coefficients, Mann–Whitney U test, and the Receiver Operating Characteristics curve. There was a difference between groups of children with adequate or better versus low MP in minutes/day of moderate PA (defined as 2,296–4,011 accelerometer counts/minute; U = 666.0, p = .045) such that those with less time engaged in moderate PA had lower MP scores than peers with adequate MP. There was also a positive correlation between moderate PA and the strength and agility MP domain (rs = .226; p = .025). In addition, minutes/day of moderate PA less than or equal to 142.56 minutes showed predictive capacity for low MP (area under the curve = 0.635; p = .027). Thus, objective measures of PA were able to predict low MP in 8-10-year-old Brazilian children, with an approximate guideline of about 2.5 hours per day of moderate PA needed to protect against low MP.